1. Field of the Invention
This invention relates to a reflection mask, and an electrically charged beam exposing apparatus used for manufacturing semiconductor devices and more particularly to an electrically charged beam exposing apparatus using a reflection mask.
2. Description of the Related Art
It has become necessary to expose patterns of less than 0.2 .mu.m on specimens in order to continue the development and research of highly technological devices employing highly developed LSIs in the future. However, it is impossible to expose such patterns on specimens by using the light exposing technology practiced in the recent production lines. Therefore, charged beam exposing apparatuses having high resolving powers, such as an electron beam exposure apparatus, are used to expose such fine patterns. In particular, relatively high throughputs are obtained by the use of electron beam apparatuses of variable shape type. The reason why such high throughputs are obtained is that electron beams are shaped as a rectangular or triangular aperture image by a shape aperture member to expose a required pattern.
FIG. 1 schematically shows how to expose resultant patterns by means of a variable shape type electron beam exposing apparatus. First rectangular aperture image 100 is projected on a second arrow-shaped aperture 110 by deflecting electron beams and the electron beams are limited to be incident on a rectangular portion 120 or triangular portions 130 which expose patterns in a specimen surface. The size of the aperture is several .mu.m to several tens of .mu.m. The electron beams selected and shaped as mentioned above are normally cut scale down to 1/20 to 1/40 and are projected on the surface of the specimen.
In the development of devices of 1G (one giga) grade, the technique as shown in FIG. 1 is impractical when exposure time is considered because the shapes of cross sections of the selected beams are limited to several shapes and further exposure must be repeated several times in order to obtain a required exposure pattern.
FIG. 2 is a schematic view of the shapes of the cross section of beams produced by the conventional electron beam exposing apparatus mentioned above in which only five limited shapes are obtained. The method of exposure will now be shown by using the cell portion of a DRAM with reference to FIG. 3. A pattern in the cell portion consists of sets of the fundamental shapes as shown in FIG. 2. A pattern is divided as shown in FIG. 3 and the divided portions are exposed in succession. The exposure of 20 shots per pattern is performed in the sample as shown in FIG. 3. Therefore, 20G shots are required for exposing the cell portion of a DRAM of 1G grade. Even when a high sensitive resist having a sensitivity of 5 .mu.C/cm.sup.2 is used at a high current density of 50 A/cm.sup.2 to shorten the exposure time, it takes 35 minutes for a chip to be exposed. In this respect, it takes about 60 hours for 100 chips on the overall area of the wafer to be exposed. Consequently, two months are required to expose one lot consisting of 24 wafers. When peripheral circuits which are arranged irregularly are included, more than four months is required to perform exposure.
This problem can be solved by performing exposure by using a shape aperture having holes of the same shape as the required exposure pattern. In the case of using an aperture which has the same shape as that of the cell of the DRAM as shown in FIG. 3, each cell is exposed by a single exposure, and the exposure time is reduced to less than 1/20 and thus exposure is made in a practical time.
The exposing method using the cell shaped aperture, however, encountered with the following difficulty:
In order to obtain electron beams of an arbitrary shape in accordance with the shape of an exposure pattern, holes having a large number of shapes and a dimensional accuracy of 0.1 .mu.m must be arranged in the shape aperture. However, the holes cannot be arranged at a practical pitch of, for example, 100 .mu.m, because the holes can not be made as donut-like openings.
Apertures are generally manufactured by forming through holes having a required shape in base material such as metal plates or silicon wafers by means of machining or an LSI process. Since distortion occurs by the machining process, the number of holes formed in an aperture member is limited to a small number such as one or two. Thus, it is difficult to form an aperture having a required shape in the base material. Further, the accuracy attained by machining process is about 0.4 .mu.m. On the other hand, by the etching technique using plasma or ions in the normal LSI process, the surface of each silicon wafer can be processed at an accuracy of about 0.1 .mu.m so as to obtain a required shape. But the depth processing is limited to several tens of .mu.m. Thus only a very thin aperture can be formed. In order to fix an aperture to a holder, the portions of the aperture other than holes must have a thickness of several hundred .mu.m and thus the holes are formed at the bottom of the wafer by means of wet etching such as the KOH method. FIG. 4 shows the cross-sectional view of apertures formed by means of the wet etching. As shown in FIG. 4, however, the etched side portion has about the same length as the etched depth, and it is difficult to arrange the holes at a pitch of 1 mm or less. Such an aperture cannot be practically used because the beams must be deflected or the aperture must be moved by 1 mm or more.
The prior art related to this invention is further disclosed in Japanese Laid-open Patent Application No. 53-29080.
The above discussion is not only applicable to electron beam exposing apparatus but also to any types of electrically charged exposing apparatus.
Since the shapes of the cross sections of the electrically charged beam is limited as explained above, the apparatus is encountered with the problem that it takes a long time to expose patterns in highly technological devices. This problem can be solved if holes having quite a large number of arbitrary shapes for shaping beams arbitrarily are arranged in the aperture member. However, it is difficult to arrange these holes accurately at a practical pitch.